Lightweight aggregate and method of producing same



Dec. m, ma. I W; J. McCoy 45mm LIGHT WEIGHT AGQREGATE AND METHOD OF PRODUCING SAME Filed Aug. 2, 194eV EAW STE HOP/JE/ 5 E l f (RUS/1f@ INKENTOR. #MQW j #6kg BY www@ t Hrm/m51 Patented Dec. 14, 1948 UITED STATS PTENT QFFICE LIGHTWEIGHT AGGREGATE AND METHOD OF PRODUCING SAME Walter d. yMcCoy, Catasauqua, Pa., assignor to Lehigh Portland Cement Company, Allentown, lla., a corporation of Pennsylvania Application August 2, 1946, Serial No. 687,903

. 11 Claims.

.l This invention relates to lightweight aggregate for use in concrete or other building material vrand to a method of making such aggregate.

The aggregates used for concrete and similar building material may be classied generally into heavy and lightweight types. The heavyweight type includes natural sands, gravels and crushed stones. Ordinary concrete usually contains this 'type aggregate and weighs around 150 pounds per cubic foot. Stress calculations ina given structure must take into account this high unit weightand therefore require the structure to be stronger than would otherwise be necessary to carry the dead load of the concrete. Lightweight aggregates, on the other hand, may be used with the result that the 'concrete is lighter per unit volume and if adequate strength is provided there is a saving in the amount of concrete required to Acarry the stresses. Concrete weighing consideryably less than 100 pounds per cubic foot and having adequate strength may be produced with aggregate made according to thisvinvention.

Lightweight aggregates have heretofore been made from shales or` clays when containing suiyci'entinitiallyv present or added quantities of sulphur, iron or carbonaceous matter to cause vesiculation upon heating. There have been `various processesA suggested for the production of lightweight aggregate based upon this reaction.

These processes, however, involved a number of operations, first in preparing the' raw material then a burning or heating operation followed by crushing and screening operations. In some of these processes the material was ground and then made into a plastic mass, pugged, extruded and then shaped to desired sized particles. That is, the plastic mass was pelletized, The pellets were then heated to causethem to expand or vesiculate. The pellets while moist were coated ,with a refractory material before heating or during heating and this formed a shell onthe finished expanded particles. The drawback'in such process is that .when the pellets are heated the refractory becomes fused and adheres to the outsideof the pellets to form a shell and this tends to defeat they primary object of producing a lightweight aggregate because the refractory being heavier than the vesiculated part of the particle adds concomitant weight to the aggregate. Nevertheless it was suggested that refractory material be used to'coat the pellets because otherwise upon heating the pellets to a temperature sufficiently 'high to cause the desired` vesiculation, they became fused together in agglomerates. In this case the agglomerates had to be broken down with vest the result that there was a loss of fines due to the crushing. Also, this left the finished aggregate with sharp corners and rough porous surfaces which are undesirable.

Other lightweight aggregates have been available such as crushed and sized clinker, or slag and the like, but these have the disadvantage of sharp corners and exposed pitted areas. These characteristics are undesirable because they are detrimental to the workability of the concrete ymix and increase the water requirement, and it is well known that excess water has a deleterious effect on the strength of concrete.

The ideal lightweight 'aggregate therefore appears to be one in whichthe discrete particles are vesiculated and of desired size or range of size and with few, or practically no, sharp corners or exposed pitted areas on the surface. At the same n sumcient to give the desired lightness of weight.

It is an object of this invention to produce a lightweight aggregate having those desired characteristics and approaching the ideal in manner and to ani extent not heretofore attained in prior art practices.

According to this invention the raw material need not be ground to produce a fine material which must be pugged and pelletized before vesiculation. On the contrary, the invention utilizes a raw material which is readily broken down to a desired size which can be vesiculated according to the method hereinafter. described without intermediate pelletizing and the process is such that the product resulting from the heating or vesiculation step is in the form of discrete particles `within the desired size range ready for use as lightweight aggregate without having to be further crushed, graded or sized. Thus, the invention eliminates a number of steps which were ,necessary according to prior practices.

Slate is the raw material from which the lightweight aggregate is produced in accordance with the invention. It contains naturally the constituents which cause it to vesiculate when properly heated. Also, it is a particularly desirable .material because of the available source, there .being vast quantities of it which havealready been quarried. Much slate now lies on waste piles ,because this slate did not lend itself to mechantation of this rejected material heretofore considered as waste.

There have been suggestions that slate be used for making an aggregate for concrete but so far as I am aware these prior suggestions have either failed completely or have had drawbacks which are overcome 'by the kprocess of this invention. O'ne of the difliculties that has not heretofore been successfully overcome arises out of the inherent characteristics of the slate itself taken together with the nature of steps which normally suggest themselves as necessary to the production of `a vesiculated or expanded slate aggregate. Since it is necessary to heat the broken up-rslate to cause it to vesiculate, it must be heated to sufficient degree and for sufficient time to obtain sulcient vesiculation to produce the -zdesired lightness of Weight. If it is heated suiiciently to obtain the required lightness this results in a fusion of the particles together, forming agglomerates which have to be fbroken lup later and this results 4in disadvantages mentioned above. n the other hand, if a temperature 'low enough touprevent fusion to eliminate agglomeration, is utilized in the'heatingstep, then theresultant product Ais not expandedsulciently toproduce the desired light- 4-ness'and the yparticles alsol do not have the undesirable sharp corners rounded off.

Itisfa further object of this invention to overcom'e 1these drawbacks whereby the slate particles'wh'ich-a ultimately to form the discrete particles of the-expanded slate aggregate maybe heated to a high enough temperature to vesiculate 'the -particles tordesiredlightness and at the same time to-fuse 'the surface suiciently to round ofi vundesirable -sharp angular cor-ners while at the same 'time undesirable fusion, lso as to cause the particles to adhere together and form undesirable agglomerates, Ais avoided.

'I-ol accomplish these desirable ends, and others mentioned hereinafter, the invention contemylplates -a `process lwhereby slate is subjected to a crushing so as 'to produce discrete raw or `un' burned slateparticles ranging in sizeup toabout ione-half lof an inch in itsflongest dimension, `or larger if desired, it being understood, of course, thatfthe yraw .slate-iis 'ordinarily laminated or in relatively' thin pieces. Dust and extreme fines yfrom .this Vcrushing operation :are preferably, although` not necessarily, removed and the pre- -sizedraw :slate :particles 'are-"then ksubjected to a heating. operation ina suitable kiln through which the particles are moved and 'meanwhile subjected .tosucient heat for sufcient .time to vesiculatevthe.particles-tothefdesired degree when Yincipient yfusion also takes place resulting lin a rounding `off of sharpcorners. When .necessary .to 'prevent the particles -frorniusing or adhering .together :during the heating operation, there is Vmixeclwith the raw pre-sized slate .particles-either before charging. theparticles to the burning `kiln or' duringr their 'passageftherethrough material .whichserves to prevent the particles from adhering together but which will ynot adhere to the vesiculated particles `-to a sufficient extent 'to substantially increase the weight -of the cooled vesiculated aggregate particles incase a particue "larly strong lightweight aggregate isdesired. The

material caused to be presentl with the slate in orderto prevent undesirable adhesion'of the-particles, one to another, during 'the heating should Abe oneth'at is granular in 'form and refractoryin nature and one that has a fusion Atemperature well above that of the slate itself. When the `slate `particles are vheated iin the presence fof such ma- 4. terial to a temperature and for suflicient time to produce a moderate amount of expansion, the material should be such that upon relatively rapid cooling of the heated slate after vesiculation only a small amount of the granular refractory material will remain adhering to the particles.

Although the n'ovel features which are believed to be characteristic of the invention will be pointed out in the annexed claims, the invention itself as to its objects and advantages and the manner in which .it .may be carried out may be better understood by reference to the following description taken in connection with the accompanying drawings forming a part hereof, in which there is showna typical flow diagram representative of the process of the invention. It will be understood that the ow sheet is diagrammatic.

Referring to the flow sheet as representing the manner of practising the invention, the Slate, if the pieces are initially ltoo large, is rst reduced to fthe desired size 'for burning :in a fkiln. 'This may be done in a hammer millor-jaw crusherfrepresented in the flow sheet at l0, although the crushing may be accomplishedbyother types of crushing devices. The slate from the crushing `device is discharged onto a screen vvlll from which ess and `produce yan excellent lightweight -aggregate `although the particles may 'be larger, yif desired, ranging lup to about an inch. Assuming a screen :l I with Aa mesh suitable to pass particles up to aboutone-half inch, then all particles of less size (-1") `will pass Athrough thescreen and are discharged to anotherscreen -M which has a ne `mesh, of the order of-6 to 8 mesh, sothat dust and extreme lines ,-pass- Lthrough and are` removed. Y

rThe ,fr-aw slate in the :form of particles which do .-not pass. through screen .-I4, that is, a range `in sizes Aup to raboutione-half inch, forms the kiln charge, it being understoodthat the vdust and-extreme fines yare preferably removed. vIf ra larger size charge for the kiln is desired the mesh .of

screen H 'may be correspondingly increased. Un-

der usual circumstances Ithe `size should not exceed an inch. i

It is preferable .not to give the slate a crushing which vis `to'o severey as 'drastic Vcrushing tends to produce Ylarger quantities 'of undesirable fdust .and extreme'nes. A lessldrasticcrushingis desirable as this has a tendency to produce more particles of the order of one-half'inch to No. '8` mesh passying to screen ll'lllan'd'it is anveasymatter to return oversize collected in the bin I2 and return Lit to the crusher for lfurther crushing.

The presized particles of slatefrom the screen I4 which vhave been freed of dust and extreme fines are passed to akiln charging hopper l 5 'from which they `may be fed into a rotary kiln lf. This may be the type of rotary kiln known Iand used for the burning of cement `and I have used the rotary type of kiln with goodresults. However, other types of lkilns may be adapted for burning the pre-sized slate particles.

Along with'ithe pre-sized slate `particles-cl'iai'gec' r'to the rotary kiln I8 there isV also fed to the kiln the refractory granular material, having the characteristics mentioned in the foregoing. This material, for convenience of description, may be called an anti-adhesion agent. As pointed out v above, the anti-adhesion agent should be granular and have a substantially higher fusion temperature than the slate particles themselves. I

Y have found a good grade of silica sand gives excel- -lent results although other granular material yhaving like characteristics, such as diatomaceous "earth, fire clay and the like may be used.

When sand is used according to the process described herein the resulting aggregate may be 'highly vesiculated and sharp corners existing on the slate particles may be fused and rounded out without fusing of the sand or anti-adhesion Y'granules inter se.

The anti-adhesion agent (herein referred to as sand for convenience of description) is fed from the sand hopper I9 to the kiln I 6 along with the pre-sized slate particles or, if desired, the same may be intermixed with the slate charge before it is fed into the kiln, when using a rotary kiln of the kind mentioned because the rotation yof the kiln causes an intermixing of sand and slate as the charge tumbles in characteristic fashion and passes down the inclined rotary kiln.

The charge is passed through the rotary kiln in the directions indicated by the arrow in the kiln of the flow sheet, counter-current to the ,flame gases, during which time the charge is subjected to a temperature between 1850 to 2250" F. A temperature of approximately 2100 F. ordinarily is best suited to produce the desired expansion or vesiculation of the particles. The burned particles are discharged at the lower end of the kiln, and passed onto a screen Il of small mesh. The hot expanded aggregate discharged from the kiln is cooled and substantially all of ythe sand in the charge passes through screen l1 and is collected in the sand bin I8 from which it maybe returned to the sand h-opper I9 along with make-up sand to be recirculated through the kiln with further slate particles being vesiculated.

While the reaction is one of time and temperature and these may have to be varied within limits depending upon variations in the particular charges being burned, I have found that the desired results may be obtained by maintaining the kiln temperature between 1850 and 2250 F. and subjecting the charge to a temperature preferably about 2100 F. for a suicient time to cause incipient fusion at the surface `of the slate particles while at the same time causing the particles to expand to three to six times their original size. The expansion which takes place is mostly in a direction at right angles to the cleavage planes of the slate.

I have obtained excellent results by burning the charge as described herein in a rotary kiln 4at about 2050 to 2100 F. for about twenty minutes while tumbling the charge of slate particles through the kiln with ten to twenty percent of silica sand by weight of the slate to keep the slate particles from fusing together during the burning or vesiculation operation. With a charge made up from a slate quarried from the mines of eastern Pennsylvaniain which the unburned slate particles ranged in size up to about -one-half inch, the aggregate treated as above described was discharged from the kiln without agglomeration and on cooling substantially none of the added sand adhered to the Vesiculated slate particles. The

"particles were vesiculated throughout, andv were rfree from sharp corners 'as the surfaces had been sufiiciently fused to round them off, and the particles ranged in size up to the order of about three-fourths inch in longest dimension. The resulting expanded aggregate weighed thirty to thirtytwo' pounds per cubic foot, according to vibrated weight procedure customarily followed in aggregate testing.

A concrete made with an expanded slate ag- "gregateproduced according to the above-mentioned procedure using a 1:2:31/2 mix (cement sand expanded slate aggregate) and contain- 'ing 6.4 sacks of cement per cubic yard, had a weight of lbs. per cubic foot in air dried condition, a compressive strength of 1880 p. s. l. at seven days and a compressive strength of 3380 p. s. i. at twenty-eight days. The sand used in this case was produced by crushing some of the expanded slate aggregate to sand size. For a similar mix when regular silica sand was used in mixing the concrete, the concrete had a weight of 92 lbs. per cubic foot in air dried condition, a compressive strength of 2160 p. s. i. at seven days and a compressive strength of 3540 p. s. i. at twenty eight days'. It may be noted here that sand used for mixingthe concrete is not to be confused with the term sand used as an anti-adhesion agent during the vesiculation step.

LightV weight concrete is frequently used for floor and roof fills, in which case high strength is not necessary and the cement content can be considerably reduced. A concrete mixture of this type in the proportions of 1:3:4% (cement ground water-cooled slag expanded slate aggregate) and containing 4.6 sacks of cement per cubic yard had a weight of 88 lbs. per cubic foot in air dried condition, a compressive strength of 900 p. s. i. at '7 days and of 1725 p. s. i. at twenty-eight lated to a greater degree and therefore lighter in weight, thismay be done by subjecting the presized particles charged to the kiln to a higher temperature for the same time or a longer time at a given temperature within the temperature range of vl8502250 F. However, there are critical factors which enter into the manipulation of the process. If the charge is permitted to become fused too much there is a tendency for the sand of such anti-adhesion agent asis used, to become permanently fused to the particles. Consequently there is a balance or compromise which should be taken into account. If the sand adheres to the expanded particles of slate this has a tendency to increase the weight. However, as the expansion or vesiculation is increased the apparent density of the particles is decreased but increased expansion has a tendency to reduce structural strength. If desired, the particles may be subjected to the time-temperature reaction in the kiln so that the slate particles are vexpanded to sufficient degree that notwithstanding the added weight attributable to added refractory material, the weight of the resulting nish-.ed aggregate may be less than 1an aggregate which lhas been produced by subjecting it to a lesser time-temperature lreaction whereby substantially none of the refractory material adheres to the particles. However, such aggregate, if over-expanded, is lacking in high structural strength, and it is not desirable for concrete which must have such strength. Nevertheless,

`such aggregate is adapted for many vuses, such,

forlexample, as a -ller Ifor sub-floors ,and roofs,or

`where insulation value rather than highstructural strength is desired.

`llt is thus seen that the invention provides a process capableof producing a wide variety of expanded slate aggregate and the process eliminates a number of operationswhich were necessary tothe production of lightweight aggregates in prier art practices, While some vVariation in temperature or time maybe required and by reason of the varying nature yof the kiln charge some trial may be necessary in order to produce anaggregate having precise characteristics of apparent `density and weight, never-theless an aggregate of desired characteristics may be pro- .duced with the temperature range of 1850 to 22.50 F. `which appears to be the critical range, andy ordinarily a temperature in the neighborhood of 2100 F. with a time reaction of about twenty minutes will produce a satisfactory result. It will be understood, however, that these factors may be varied to suit particular situations at hand and these will be dependent largely upon the characteristics desired in the finished product.

As, examples of aggregates produced according to the process of the invention wherein the kiln charge in each instance was made up of minus one-half (-1") inch slate particles and about 20% by weight of silica sand, the resulting aggregate froin one such charge when subjected to a temperature in a rotary kiln approximately 2050 to 2l00 F. for about twenty minutes, weighed 30 to 32 .pounds per cubic foot and was substantially free of adhering sand; another charge when subjected to a temperature approximating 21'00- 2135 F. for about 20 minutes was expanded more and had a weight of about 28 pounds per cubic foot vbut also had permanently adhering thereto a small amount of sand which added concomitantly to the weight yet the sand particles were not fused inter se; and a third charge when subjected. to a temperature approximating 2l35 to 21.70 F. for about 20 `minutes was expanded more than the second chargel mentioned above, but also `had `a considerable quantity of sand adhering to the particles and had a weight of about 26 pounds -per cubic foot but the sand granules were not fused inter se; and a fourth charge subjected to a Atemperature approximating 21702200 F. Vfor about 20 minutes resulted in an even greater expansion of the particles but had more sand permanently adhering thereto and had a weight of 24 .pounds per cubic foot, but the structural strength was too low for use in a concrete requiring high strength. The foregoing examples are kindicative to show the nature of the process and .to show how the process may be manipulated yte best advantage.

It will thus be apparent from the preceding disclosure that the invention provides a simple process of producing a lightweight aggregate which eliminates a number of operations which prior practices required, and the discrete particles may be vesiculated substantially uniformly through- '8 `out andk they may be caused to ybe free yof surface cracks and jagged projections, yet at no time is the aggregate pulverized either before or after vesiculation. The raw materialsare economical lto use and -a wide variety of light aggregates of desirable characteristics may be made from an otherwise wasted product. While it is in most instances highly important to vesiculatel the slate charge in the ,presence of an anti-adhesion agent, it may not in some instances be necessary. 'Whereall'the benefits of this now preferred practice are not necessary or desirable, the vesiculation step might be rcarried out in the absence of such agent. Also, in some instances, the quantity of anti-adhesion -agent may be varied. However, if :larger quantities are used it results in less efficient `operation as the heating of the excess requires more fuel without corresponding increase in quality of nished aggregate.

'Ihe terms Aand expressions which have been employed herein are used as terms of description and not 'of limitationand there is no intention in the use oi such terms and expressions of excluding any equivalent of the features shown and described or portions thereof, but it is recognized that various modications are possible Within the scope of invention claimed.

What is claimed is:

.1. A lightweight aggregate suitable for making concrete `which comprises discrete particles of expanded vesiculated :burned unpulverized slate particles substantially all of which are less than one inch in longest dimension, said discrete particles being substantially free from sharp angular corners and exposed pitted surfaces, said aggrelgate weighing not substantially more than thirtyfive pounds per cubic foot.

2. A lightweight aggregate made from presized slate pieces suitable for making yconcrete which .aggregate comprises discrete vesiculated slate` particles vexpanded three to six times their initial size, substantially all of said expanded particles vranging in size upto the order of threefourths finch in longest dimension, said particles having initially contained sharp corners fused and rounded oi and having surfaces substantially free from cracks, jagged projections and exposed pitted areas, said aggregate weighing less than thirty-nire pounds per cubic foot.

3. A light yweight aggregate made from presiz-ed unpulverized raw slate particles and suitable for :makingconcrete which comprises discrete particles ol :expanded vesiculated burned unpulverized slate lparticles substantially all of which are less than Aan .inch in longest dimension, .said discrete particles being expanded three to six times their initial size and substantially free from sharp yangular corners and exposed pitted surfaces and weighing `less than thirty-ve pounds per cubic foot.

4. A method of 4producing a lightweight aggregate suitable for making concrete which comprises burning presized slate particles in a kiln in the Ypresence'oi a coarse silicasand material capable of preventing adhesion of said particles when said particles are heated high enough `to fuse while agitati-ng the particles for a suflicient time and at a temperature sufficiently high to expand yand vesiculate the particles and to cause suicient fusion of the particles to round `orf existing sharp corners and removing the expanded particles from the kiln as a lightweight aggregate fin the -orrn of vdiscrete particles substantially `free of agglomerates.

:5. A method of producing a lightweight aggre- 9. gate the discrete particles of which are substantially free from jagged projections which comprises burning a charge of presized slate particles in a kiln While tumbling the particles in the presence of silica sand while subjecting the charge to a temperature between 1850 F. and 2250 F. and for a time suicient to vesiculate the slate particles and to fuse and round off sharp corners that may be present in the charged particles but nsuicient to cause substantial fusion inter se of the grains of silica sand.

6. A method of producing a light Weight and vesiculated slate aggregate suitable for making concrete which comprises providing unburned presized particles substantially free from pulverulent fines ranging in size up to a length of the order of one-half inch, tumbling the said particles in a kiln in the presence of silica sand while subjecting the charge of slate and sand to a temperature between 185 F. and 2250 F. for a length of time sufficient to vesiculate the slate particles and expand them three to six times their initial size without causing a substantial amount of said sand to become permanently fused to the expanded particles so produced.

7. A method of producing a light weight and Vesiculated slate aggregate suitable for making concrete which comprises providing unburned presized particles substantially free from pulverulent fines ranging in size up to a length of the order of one-half inch, tumbling the said particles in a kiln 'in the presence of silica sand in the amount of to 20% by weight of the slate charge while subjecting the charge of slate and sand to a temperature of about 2100o F. for about twenty minutes to vesiculate t-he slate particles and expand them three to six times their initial size without causing a substantial amount of said sand to become permanently fused to the expanded particles so produced.

8. A method of producing a light weight aggregate suitable for making concrete which comprises providing a charge of presized raw slate particles, substantially free of pulverulent nes, substantially all of the particles of which are of a size less than the order of one-half inch in longest dimension, passing the charge through a kiln maintained at a temperature between 1850 F. to 2250 F. While tumbling the charge in the kiln in the presence of a granular silica sand refractory material having a fusion temperature higher than the slate charge and in sufficient amount to prevent agglomeration of said particles and maintaining the charge in said kiln for suicient time to vesiculate and expand the slate particles three to six times their initial size.

9. A method of producing a light weight aggregate suitable for making concrete which cornprises providing a charge of presized raw slate particles, substantially free of pulverulent nes, substantially all of the particles of which are of a size less than the order of one-half 'inch in longest dimension, passing the charge through a kiln maintained at a temperature between 1850o to 2250o F. while tumbling the charge in the kiln in the presence of silica sand in sufficient amount to prevent agglomeration of said particles and maintaining the charge in said kiln for sufficient time to vesiculate and expand the slate particles three to six times their initial size.

10. A method of producing a light weight aggregate suitable for making concrete which comprises providing a charge of presized raw slate particles, substantially free of pulverulent fines, substantially all of the particles of which are of a size less than the order of one-half inch in longest dimension, passing the charge through a rotary kiln maintained at a temperature of about 2100 F. While tumbling the charge in the kiln in the presence of 10% to 20% of silica sand by weight of the slate charge and maintaining the charge of slate and sand in said kiln for about twenty minutes to vesiculate and expand the slate particles three to six times their initial size.

11. A method of producing a light weight aggregate suitable for making concrete which comprises providing a charge of presized raw slate particles, substantially free of pulverulent nes, substantially all of the particles of which are of a size less than the order of one-half inch in longest dimension, passing the charge through a rotary kiln maintained at a temperature between 1850 and 2250 F. while tumbling the charge in the kiln in the presence of 10% to 20% of silica sand by weight of the slate charge and maintaining the charge of slate and sand in said kiln for suilicient time to vesiculate and expand the slate particles three to six times their initial size, thereby to produce a light weight vesiculated slate aggregate weighing less than thirty-five pounds per cubic foot, substantially all the particles of which are less than the order of three fourths inch in longest dimension and free from jagged projections and exposed porous surface areas.

WALTER J. MCCOY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 930,801 Senn et a1 Apr. 1, 1909 2,021,956 Gladney Nov. 26, 1935 2,103,746 Guth Dec. 28, 1937 2,199,046 Evenstad Apr. 30, 1940 2,265,358 Denning Dec. 9, 1941 OTHER REFERENCES Chemical Abstracts, vol. 25, page 314'7, Shablukin, et al.; ibid., vol. 36, page 3925,l Platzmann.

Certificate of Correction Patent No. 2,456,207. December 14, 1948.

WALTER J. MCCOY It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 9, line 20, claim 6, for 185 F. read 1850 F.;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 12th day of April, A. D. 1949.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

